FIG. 1 illustrates an exemplary photoplethysmography (PPG) signal 102 produced using a PPG device (also known as a PPG sensor). For timing reference, an electrocardiogram (ECG) signal 104 is also illustrated. The PPG signal 102 can be used to measure the volume of arterial and venous vasculature. Additionally, a measure of arterial pulse amplitude can be derived from the PPG signal 102. A few tens to a few hundreds of milliseconds after the QRS complex of the ECG signal 104, the PPG waveform reaches a minimum and starts to increase. This is due to the increasing blood volume in the arterioles as the systolic pulse reaches the periphery. The delay is influenced by the distance that the PPG sensor is placed from the heart. It requires approximately 100 msec for the waveform to reach its maximum. The excursion from minimum to maximum represents the arterial pulse amplitude. During diastole, the recoil of the elastic arterial vessels continues to force blood through the capillaries, so that blood flows through the capillary bed throughout the entire cardiac cycle.
A PPG sensor (also called a pseudoplethysmography or photoelectric plethysmography sensor) includes a light source and a light detector. The PPG sensor utilizes the transmission or reflection of light to demonstrate the changes in blood perfusion. Such devices might be used, e.g., in the cardiology department or intensive care department of a hospital or in a clinic for diagnostic purposes related to vascular surgery.
A block diagram of an exemplary PPG sensor is shown in FIG. 2A. An exemplary mechanical arrangement for a noninvasive (i.e., not implanted) PPG sensor is shown in FIG. 2B. An exemplary mechanical arrangement for a chronically implantable PPG sensor is shown in FIG. 2C.
The PPG sensor includes a light source 206 and a light detector 214. In one example, the light source 206 includes one or more light-emitting diode (LED), although in alternative models an incandescent lamp or laser diode can be used as the light source. Referring to FIG. 2A, the light source 206 outputs a transmit light signal 208 that is transmitted through and/or reflected by (depending on the embodiment) patient tissue 210. For example, light may be transmitted through a capillary bed such as in an earlobe or finger tip. As arterial pulsations fill the capillary bed and pre-capillary arterioles, the changes in volume of the blood vessels modify the absorption, reflection and scattering of the light. Stated another way, an arterial pulse in, for example, a finger tip, or earlobe, causes blood volume to change, thereby changing the optical density of the tissue. Therefore, the arterial pulse modulates the intensity of the light passing through the tissue.
A receive light signal 212 is received by the light detector 214. The light detector 214 can include, for example, a photodiode. Changes in light intensity cause proportional changes in the photodiode current, which can be converted to a varying analog voltage light detection signal 216 by a transimpedance amplifier. The light detector can, for example, alternatively include a photoresistor, phototransistor, photodarlington or avalanche photodiode. Light detectors are often also referred to as photodetectors or photocells.
PPG sensors may operate in either a transmission configuration or a reflection configuration. In the transmission configuration, the light source 206 and the light detector 214 face one another and a segment of the body (e.g., a finger or earlobe) is interposed between the source 206 and the detector 214. In the reflection configuration, the light source 206 and the light detector 214 are mounted adjacent to one another, e.g., on the surface of the body, as shown in FIG. 2B. In this configuration, a fraction of light from the light source 206 is backscattered by the tissue into the light detector 214.
Referring to FIG. 2C, if the PPG sensor is incorporated into a chronically implantable device 220 (e.g., an implantable cardioverter defibrillator (ICD), pacemaker, or any other implantable device), the light source 206 and the light detector 214 can be mounted adjacent to one another on the housing or header of the implantable device. The light source 206 and the light detector 214 are preferably placed on the side of the implantable device 220 that, following implantation, faces the chest wall, and are configured such that light cannot pass directly from the source to the detector. Thus, the reflection configuration is preferably used when the plethysmography device is implemented in an implantable device. The placement on the side of the device 220 that faces the chest wall maximizes the signal to noise ratio by 1) directing the signal toward the highly vascularized musculature, and 2) shielding the source and detector from ambient light that enters the body through the skin. Alternatively, at the risk of increasing susceptibility to ambient light, the light source 206 and the light detector 214 can be placed on the face of the device that faces the skin of the patient. Additional details of an implantable PPG device are disclosed in U.S. Pat. No. 6,491,639, entitled “Extravascular Hemodynamic Sensor” (Turcott), which is incorporated herein by reference.
The varying analog voltage light detection signal 216 that is produced by the light detector 214 is a PPG signal. The PPG signal is typically filtered, amplified and converted to a digital signal using an analog to digital (A/D) converter (not necessarily in the order). For example, the signal may be sampled at 500 Hz (i.e., 500 samples per second) using a high resolution A/D converter, and then the samples may undergo relatively intensive post-acquisition filtering (e.g., using a 1000-point digital filter). This relatively high sampling rate and relatively intensive filtering consume battery power and processing resources. While this may not be much of a concern with a non-implanted PPG device (e.g., such as the one shown in FIG. 2B), minimizing power consumption and processing is very important when it comes to implantable devices. This is in part because invasive surgery is required to replace the battery of an implanted device. Accordingly, there is a desire to reduce, and hopefully minimize, both the number of samples that are acquired, and the associated processing of such samples. Additionally, there is a desire to reduce the amount of power that is required to produce and process the samples.